1. Field Of The Invention
The present invention relates to a gas doser to be used in a gas dosing method wherein measuring conduits are filled with a calibrating gas, and its flow passage is changed by a valve whereby a certain quantity of correcting gas of known concentration can be caused to flow into a gas detecting instrument to correct (recalibrate) the instrument.
2. Description Of The Prior Art
In a gas detector such as a gas analysis meter, it is usual to correct (recalibrate) the detector periodically in order to maintain its detecting precision. As one calibration method, it is known to use a gas dosing method wherein a certain quantity of calibrating gas of known concentration is caused to flow into the gas detecting instrument using a carrier gas.
The conventional gas doser previously used in this method is explained with reference to prior art FIGS. 5(a) and 5(b). In a change-over valve A having six change-over ports P.sub.1 -P.sub.6, a first input port P.sub.4 for calibrating gas communicates with connecting port P.sub.3 and a measuring pipe B, and calibration gas is caused to flow via the input port P.sub.4, port P.sub.3, measuring pipe B, port P.sub.6, port P.sub.5, and outlet EXH in order to fill measuring pipe (tube) B with calibrating gas as shown in FIG. 5(a). Measuring pipe B has been designed previously with a prescribed volume. Accordingly, the volume of calibrating gas filling measuring pipe B is a known quantity. Next, valve A is changed to concurrently block communication between input port P.sub.4 and connecting port P.sub.3 and provide communication between port P.sub.3 and port P.sub.2. As a result, carrier gas is fed into port P.sub.3 from port P.sub.2 as shown in FIG. 5(b), whereby this carrier gas is introduced into measuring pipe B and calibrating gas within measuring pipe B is fed into the detector from port P.sub.6 via port P.sub.1 by this carrier gas.
However, in this conventional apparatus, "dead space" C (the part disclosed by oblique lines in FIG. 5 (a) (b)) is formed at the connecting part and change-over part of measuring pipe B, which, unless considered, leads to a measuring error. It is therefore necessary to count the previous content of this dead space. However, it has previously been found troublesome to count this dead space.
Further, in the gas detector multiple point calibration with varied calibrating gas concentrations has been made necessary, because, as is illustrated in the graph of detector output versus calibrating gas concentration shown in FIG. 6, even if a correct detector output corresponding to a particular calibrating gas concentration was obtained at point I, if a correct output value was not obtained for point II or point III where the calibrating gas concentration has been varied there is no sense in making the correction. Accordingly, it is necessary to perform multiple point calibration with varied calibrating gas concentrations. However, in the structure illustrated in FIGS. 5(a) and 5(b), since the quantity of calibrating gas which fills measuring pipe B is constant, it is necessary to perform the laborious task of successively substituting a number of pipes B having differing internal volumes in order to obtain multiple point calibration.